In electrophotography an image comprising an electrostatic field pattern usually of non-uniform strength (also referred to as an electrostatic latent image), is formed on an insulative surface of an electrophotographic element comprising at least a photoconductive layer and an electrically conductive substrate. Several types of electrophotographic recording elements are known for use in electrophotography. In many conventional elements, the active photoconductive or charge-generation materials are contained in a single layer. This layer is coated on a suitable electrically conductive support or on a non-conductive support that is overcoated with an electrically conductive layer. In addition to single-active-layer electrophotographic recording elements, various multi-active electrophotographic recording elements are known. Such elements are sometimes called multi-layer or multi-active-layer elements because they contain at least two active layers that interact to form an electrostatic latent image.
Among the many different kinds of photoconductive materials which have been employed in the aforementioned single-active layer and multi-active elements are titanyl phthalocyanine pigments such as titanyl phthalocyanine pigment or titanyl tetrafluorophthalocyanine pigment. Electrophotographic recording elements containing such pigments as charge-generation materials are useful in electrophotographic laser beam printers because they are capable of providing good photosensitivity in the near infrared region of the electromagnetic spectrum, i.e. in the range of 700-900 nm.
Unfortunately, electrophotographic recording elements of the prior art which contain photoconductive titanyl phthalocyanine pigments have typically suffered from one or more disadvantages that have significantly restricted their use. For example, as indicated in U.S. Pat. No. 4,701,396, issued Oct. 20, 1987, photoconductive titanyl phthalocyanine pigments are not readily dispersible in liquid coating compositions comprising solvent solutions of polymeric binders which are used to dispersion coat charge generation layers in electrophotographic recording elements. It is necessary that the titanyl phthalocyanine pigment be in a form, (crystalline or amorphous) that is highly photoconductive and sufficiently and stably dispersed in a coating composition to permit its being applied at a low enough concentration to form a very thin layer having high electrophotographic speed in the near infrared range.
In U.S. Pat. No. 4,701,396, the titanyl phthalocyanine pigment is subjected to a treatment which modifies its crystalline form and reduces its particle size so that the pigment can be dispersed in liquid coating compositions comprising a solvent solution of polymeric binder. This treatment is called "acid-pasting" which involves dissolving the titanyl phthalocyanine pigment (after extraction purification of the as-synthesized material) in cold, concentrated mineral acid, preferably sulfuric acid, and pouring the solution into ice water to re-precipitate the pigment. The precipitate is washed free of acid with water, then with an alcohol and dried. The resulting titanyl phthalocyanine pigment has a substantially smaller particle size (slightly less than 1 micrometer) than the crude pigment and is highly sensitive to radiation in the near infrared range. In commercial scale operations it is, of course, desirable to avoid using large amounts of concentrated mineral acids such as sulfuric acid because of safety and environmental considerations. It is also very costly to provide the necessary safeguards for handling such a hazardous material.
Japanese Patent Application No. 117437/86, filed May 23, 1986 (Published Unexamined Application [Kokai] No. 62/275272, published Nov. 30, 1987) describes an electrophotographic recording element that comprises a layer of a titanyl phthalocyanine pigment dispersed in a polymeric binder and exhibits sensitivity in the infrared range. The titanyl phthalocyanine pigment is an amorphous material having a particle size no more than 0.2 micrometer and exhibits no strong X-ray diffraction peaks. To achieve this particle size a chemically treated pigment, e.g. an acid pasted pigment is ground by mechanical means to reduce its particle size to no more than 0.2 micrometer.
This invention is directed toward the objective of providing electrophotographic coating compositions comprising finely-divided titanyl phthalocyanine pigments that are stably and uniformly dispersed therein and to electrophotographic recording elements prepared with such compositions. It is also an objective of this invention to provide such compositions without the need for chemical treatments such as acid-pasting.